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Coral reefs in the region of the Equator do not have to contend with frequent cyclonic storms. "Hot spots" of storm activity are found outside the equatorial zone at individual sites such as Guam, where storms are frequent and violent and the coral reefs tend to have high diversity but a low profile. By contrast, reefs in Micronesia, French Polynesia, Thailand or the southern Caribbean, for example, fall victim to such storms less than once in 50 years. These rare-storm reefs tend to develop more fragile coral communities of highly complex structure. So changes in the frequency and force of cyclonic storms can be expected to have major effects on the structure and growth rates of coral reefs.
Wave action strongly influences the depth to which reef corals, algae and other key organisms are distributed. Storms provide catastrophic pruning of reefs and substrate renewal. This makes it easier for new corals to recruit from elsewhere and thus influences the way the community is made up or operates. Storms, waves and currents also mobilise sediment and nutrients as well as shaping the coastline, and affect local sea-level. Currents play a major part in transporting pathogens and nutrients and in dispersing larvae that control the distribution of reefs. So changes to these physical parameters could alter the distribution of reefs and reef organisms.
Freshwater runoff or groundwater discharge affect reef development adjacent to land masses. Reef communities need a fairly stable range of salinity. If the salinity drops below 20 parts per thousand (ppt) (normal sea-water is 32ppt) for more than 24 hours, then corals and many other reef organisms die. Salinity variations that damage nearshore reefs could become more frequent if storms, rainfall and flooding increase.
Ultra-violet radiation is known to have detrimental effects on reef organisms.
Most adult corals and shallow water reef animals and plants have developed mechanisms to either block out or avoid this harmful radiation. However, the larval stages of reef organisms are vulnerable, during their planktonic stages, to increased ultraviolet light.
Increased sea-surface temperatures or dissolved nutrients from freshwater run-off could ' affect coral reef plankton and encourage the growth of phytoplankton. Increased phytoplankton, the food of larval crown-of-thorns starfish, may result in increased populations of this harmful species that has devastated many reef systems in the past.
Increased carbon dioxide concentrations affect the acidity of the ocean's surface waters and reduce the amount of dissolved calcium carbonate available to reef-building corals.
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